The present invention relates to a cathode ray tube comprising an envelope within which is provided a channel plate electron multiplying structure disposed between electron producing means and an output device the electron multiplying structure comprising a stack of n apertured, substantially planar dynodes, the dynodes being separated from each other by spacing means and being arranged in cascade with the apertures in adjacent dynodes being aligned to form channels.
The present invention also relates to a channel plate electron multiplying structure for use in cathode ray tubes as well as other tubes such as photomultiplier tubes.
British Patent Specification No. 1434053 discloses a discrete electrically conductive dynode of perforate metal sheet form, which dynode is usable in an electron multiplying structure of the type described. The known dynode has an array of apertures which produce electron multiplication through secondary electron emission and which, viewed axially through the thickness of the dynode, are of re-entrant shape, for example concave, such that the input and output cross-sections at the opposite surfaces of the dynode are smaller than that midway through the thickness of the dynode. As it is difficult to make re-entrant shaped apertures by conventional etching techniques, it is customary to make dynodes from two sheets having generally convergent apertures therein and arrange them back-to-back so that the surfaces into which the larger diameter apertures open are in contact with each other.
In order to make a multiple stage electron multiplier then a plurality of such dynodes are arranged as a stack, with the dynodes being separated from each other by a spacing member but with the apertures in the dynodes aligned. The input dynode may be a sheet forming a half dynode and similarly a half dynode may be arranged at the output to form a focusing electrode or accommodation for colour selection electrodes.
As a general rule the input and output cross-sections of the apertures in a dynode are substantially the same and correspond to thickness of a dynode. Thus for example a dynode having apertures at a pitch of 770 .mu.m, has re-entrant shaped apertures with input and output cross-sections of 300 .mu.m and a dynode thickness of 300 .mu.m which means each sheet of the two sheets forming a dynode is 150 .mu.m thick. Such dynodes are reasonably easy to handle and are fairly rigid when assembled as a stack to form a channel plate electron multiplier structure.
In the case of using a laminated dynode electron multiplier as part of a display device, the resolution of the image is dependent upon the pitch of the apertures in the dynodes. In the case of say a display tube having a screen of 300 mm diagonal then ideally the pitch of the apertures should be of the order of 250 .mu.m and the input and output cross-sections of the apertures should be of the order of 100 .mu.m which means that the dynode thickness should be 100 .mu.m and the sheet thickness 50 .mu.m. Sheets and dynodes of such thickness are difficult to handle and also the laminated dynode electron multiplier will not be so rigid and may suffer from microphony.